Active carbons with a BET specific surface area greater than 1,500 m.sup.2 /g, preferably greater than 2,000 m.sup.2 /g, are useful for various purposes, such as fractionation of hydrocarbons, purification of industrial gases, adsorptive removal of toxic gases, antipollution measures, liquid phase purification in food and chemical industries, water treatment, recovery and fractionation of liquid phases, catalysts or catalyst supports, electric double layer capacitors, and so on.
For producing such high surface area active carbons, the following potassium hydroxide activation process is disclosed in U.S. Pat. No. 4,082,694 (Japanese Patent Publication No. 62-61529; Japanese Kokai Tokkyo Koho No. 52-92894). In said process, a carbonaceous material comprising coal coke, petroleum coke or a mixture thereof is mixed with granular hydrous potassium hydroxide, the mixture is dehydrated by heating in a precalciner within a temperature range between about 315.6.degree. C. (600.degree. F.) and about 482.2.degree. C. (900.degree. F.) and the product of precalcination step is then heated in a calciner to a temperature between about 704.4.degree. C. (1,300.degree. F.) and about 982.2.degree. C. (1,800.degree. F.), whereby activation is effected to give active carbon with a high surface area greater than 2,300 m.sup.2 /g. An example is given there in which a rotary precalciner equipped with counter rotating auger was used as the precalciner. As regards the calciner, the specification mentions to the effect that an indirectly fired rotary calciner is preferred, without any further comment.
In U.S. Pat. No. 5,064,805 (Japanese Kokai Tokkyo Koho No. 02-97414), a method of producing high quality activated carbon is disclosed which uses coconut shell char as a carbonaceous material and potassium hydroxide hydrate as an activator and in which a mixture of these is heat-treated at a temperature conducive to activation. The apparatus used in the examples is a vertical calciner equipped with a stirrer, heating means and nitrogen inlet means.
PCT International Publication WO 91/12203 (International Application No. PCT/JP/00127) discloses a carbonaceous material with high electrostatic capacitance as prepared by heat-treating an activated carbon material at a temperature lower than 700.degree. C. in an alkali metal hydroxide bath. The activated carbon material includes coconut shells, wood dust, coal, resins, etc. The alkali metal hydroxide is the hydroxide of sodium, potassium, lithium or the like. No apparatus is mentioned in the specification, however.
Japanese Kokai Tokkyo Koho No. 63-78514 discloses a double-layer capacitor comprising a polarizing electrode of activated carbon, which is prepared from a petroleum coke and has a large specific surface area, in combination with an electrolyte of an aqueous solution. The large specific surface area activated carbon is prepared by activation of the petroleum coke. However, the specification does not mention any apparatus for the production of such activated carbon.
Japanese Kokai Tokkyo Koho No. 03-294780 and No. 03-294781 disclose a material for conveying rails for an alkaline-atmosphere firing furnace and a material for a lining of an alkaline-atmosphere firing furnace, respectively. A pusher-type tunnel kiln is given as an example of the alkaline-atmosphere firing furnace. As shown in the examples, the alkaline-atmosphere firing furnace is, for example, a "fluorescent pigment which, upon heating, generates Na.sub.2 O, an alkali metal sulfide and so forth."
In U.S. Pat. No. 4,082,694 (Japanese Patent Publication No. 62-61529), a two-step process is disclosed which gives high surface area active carbons by the potassium hydroxide-based activation method using rotary calciners. However, this process still has certain problems, such as adhesion of the reaction mixture to the inside wall of the apparatus, possible sealing failure because of the presence of rotary members, and relatively poor heat economy because of being a two-step process. In particular, sealing failure may result in catching fire or explosion due to the product gas (hydrogen gas) or vaporized metallic potassium, hence it may become a fatal defect in the commercial practice of the process.
The vertical calciner used in U.S. Pat. No. 5,064,805 (Japanese Kokai Tokkyo Koho No. 02-97414) is operated batchwise and therefore is unacceptable from the commercial viewpoint although it can be used in laboratories.
The high surface area active carbons obtained by the processes disclosed in the above-cited references are very excellent in quality and therefore it is earnestly desired that they be made available on the market. Regrettably, however, any commercial-scale apparatus has not been developed as yet for stable production of said active carbons.
The carbonaceous material with high electrostatic capacitance disclosed in International Publication WO 91/12203 and the double-layer capacitor disclosed in Japanese Kokai Tokkyo Koho No. 63-78514 are nothing but an example of the applicability of the alkali metal hydroxide-based activation method. The firing furnaces disclosed in Japanese Kokai Tokkyo Koho No. 03-294780 and No. 03-294781 merely provide an example of the pusher-type tunnel kiln and remain within the scope of the background art.
Under these circumstances, it is a primary object of the invention to provide a safe and commercially advantageous apparatus for producing high surface area active carbons by the alkali metal hydroxide-based activation method.